1. Field of the Invention
This invention relates generally to a system and method for controlling stack cathode inlet relative humidity (RH) and, more particularly, to a system and method for controlling stack cathode inlet RH when RH sensing devices are not functioning properly to prevent improper humidification of the fuel cell stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, a gas diffusion layer (GDL) that protects the mechanical integrity of the membrane and helps in uniform reactant and humidity distribution. The part of the MEA that separates the anode and cathode flows is called the active area, and only in this area the water vapors can be freely exchanged between the anode and cathode. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates (separators) positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors (flow fields) for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Fuel cell membranes are known to have a water-uptake which is necessary to provide proton conductivity. The water-uptake behavior of fuel cell membranes, however, causes an increase in the volume of the membranes if conditions become more humid or wet and a decrease of the volume if conditions become dryer. Changes in the volume of the cell membranes may cause mechanical stress on the membrane itself and the adjacent fuel cell components. In addition, a membrane that is too wet may cause problems during low temperature environments where freezing of the water in the fuel cell stack could produce ice that blocks flow channels and affects the restart of the system. Membranes that are too dry may have too low of an electrical conductivity at the next system restart that affects restart performance and may reduce stack durability.
It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas of a fuel cell stack, and use the water to humidify the cathode input airflow. It is also known in the art to use relative humidity (RH) sensors to monitor the humidification of the cathode input airflow. However, RH sensors can be unreliable and can fail. Therefore, there is a need in the art to provide a method for maintaining an appropriate level of cell membrane humidification when the RH sensors are not functioning properly, as evidenced by invalid RH sensor readings, to improve stack performance by reducing the chance of liquid water occurring, extending the life of the stack membranes and by increasing stack durability.